Evidence Synthesis

Evidence synthesis is the umbrella concept for bringing together the findings of multiple studies to answer a question, of which the systematic review with meta-analysis is the best-known form. It spans a family of review designs — qualitative and quantitative, aggregative and configurative — chosen to match the kind of question being asked and the nature of the available research, and it provides the methodological foundation for evidence-informed decisions in health policy and practice.

Definition

Evidence synthesis is the process of systematically locating, appraising, and combining the results of multiple studies — quantitatively, qualitatively, or both — to produce an integrated answer to a defined question.

Scope

This topic surveys the landscape of synthesis: how review designs differ, when aggregative pooling versus configurative interpretation is appropriate, how qualitative and mixed-method evidence is combined, and how the certainty of synthesised evidence is judged. It frames evidence synthesis as a methodological field; specific quantitative procedures are detailed in sibling topics.

Core questions

• What type of review best answers the question at hand?
• Is the aim to aggregate comparable findings or to configure and interpret diverse evidence?
• How can qualitative and quantitative evidence be combined responsibly?
• How should the trustworthiness or certainty of the synthesis be conveyed?

Key concepts

• Review typology
• Aggregative versus configurative synthesis
• Qualitative evidence synthesis
• Mixed-methods synthesis
• Realist and narrative synthesis
• Certainty of synthesised evidence (GRADE / GRADE-CERQual)
• Fitness of design to question

Mechanisms

Synthesis methods share a common backbone — an explicit question, systematic searching, appraisal, and integration — but differ in how they combine evidence. Aggregative reviews pool comparable quantitative results to estimate an effect; configurative reviews arrange diverse, often qualitative, findings to generate or test an explanation. Choosing among them depends on the question and the evidence: a tightly defined effectiveness question favours aggregative synthesis, whereas a question about meaning, process, or theory favours configurative approaches. The certainty of the output is then judged with frameworks such as GRADE for quantitative effects (gough-2012; grant-booth-2009; munn-2018; guyatt-2008-grade).

Clinical relevance

Evidence synthesis supplies the integrated evidence that guidelines, commissioners, and health technology assessment bodies rely on. Recognising which synthesis design produced a piece of evidence helps a reader judge its scope and limits. These methods describe how bodies of evidence are assembled and graded; they are not direct instructions for individual care.

Evidence & guidelines

Methodological guidance distinguishes review designs and matches them to question types: typologies catalogue the available approaches, decision guidance separates systematic from scoping reviews, the Cochrane Handbook codifies aggregative intervention synthesis, and GRADE structures certainty rating (grant-booth-2009; gough-2012; munn-2018; higgins-handbook-2019; guyatt-2008-grade).

History

The idea of formally combining studies grew from mid-twentieth-century statistics into a broad methodological field as systematic reviews spread in the 1990s. As reviewers confronted questions that pooling alone could not answer, a wider family of designs — narrative, realist, qualitative, and mixed-methods synthesis — was articulated, and typologies in the 2000s and 2010s mapped this diversity and tied each design to the questions it suits (grant-booth-2009; gough-2012).

Debates

Can qualitative and quantitative evidence be validly combined?

Mixed-methods and configurative synthesis promise richer answers but raise questions about commensurability and rigour; methodologists continue to develop standards for combining different evidence types transparently.

Key figures

• David Gough
• James Thomas
• Andrew Booth
• Zachary Munn
• Gordon Guyatt

Seminal works

• gough-2012
• grant-booth-2009
• guyatt-2008-grade

Frequently asked questions

Is evidence synthesis the same as a systematic review?

Not quite. A systematic review is one well-defined form of evidence synthesis; the broader term also covers scoping, narrative, qualitative, realist, and mixed-methods syntheses chosen to fit different questions.

What does aggregative versus configurative synthesis mean?

Aggregative synthesis pools comparable findings to estimate an effect, while configurative synthesis arranges and interprets diverse findings to build or test an explanation; many reviews combine elements of both.